Oil shouldn't be gray. It might have a lot of metal floating around in it. The Auburn is well known for this. As the Auburn wears the cones and differential case put a lot of powdered metal into the oil. I had a picture that I would post in a case like this, but I lost all of my pictures when Webshots sold out. The picture shows me dipping my finger into the bottom of a Chevy rear end housing with a worn out Auburn differential. The metal sludge in the bottom of the housing was about 3/8 inch deep.